DESCRIPTION: Beside their well established role in the production of massive concentrations of toxic hydrogen sulfides, sulfate-reducing bacteria have proven useful since they contain proteins whose structure is similar to domains of more complex enzymes which are found in mammals and where they are responsible for important biological functions. Recently, the exploration of the system that allows the reduction of oxygen to water in these so-called "strict anaerobes" has culminated with the discovery of a protein that contains Fe-Uroporphyrin I as a prosthetic group. This compound, which accumulates in large amounts in some genetically-linked porphyrias, was so far considered as a non-metabolite. This discovery, in addition to providing a new physiological way of reducing oxygen to water and of showing for the first time a role for rubredoxin (Rd) in an anaerobe, has raised the possibility that Uroporphyrin I may play important roles in other organisms. The specific aims of the proposed research are: 1) to determine the structure of two of the three key components of the enzymatic complex that allows the reduction of oxygen to water with NADH as electron donor (the three-dimensional structure of the third, namely Rd, is already known to a resolution below 1 A); 2) to examine in detail the electron transferring properties of the NAD-Rd-oxidoreductase (NRO); 3) to delineate the interacting domain between Rd and NRO; 4) to study the interplay between the three redox components of the Rd-Oxygen-oxidoreductase (ROO); 5) to determine the chemical structure of the heme c-binding component of ROO.